1. Field of the Invention
The present invention relates to a voltage controlled oscillator apparatus having voltage controlled oscillators mainly of an LC-resonant type, and particularly to a broadband version thereof.
2. Description of the Related Art
FIG. 15 is a circuit diagram of a conventional LC-resonant voltage controlled oscillator 30. This circuit includes a three-terminal inductor 31, a band-switching capacitor 32, a continuously variable capacitor 33 and n-channel transistors 34. The oscillation frequency of this voltage controlled oscillator 30 is represented by Equation (1) below.
                              f          0                =                  1                      2            ×            π            ⁢                                          L                ×                C                                                                        (        1        )            Here,                C: resultant capacitance of the band-switching capacitor 32 and continuously variable capacitor 33        L: inductance of the three-terminal inductor 31        
The capacitance value of the continuously variable capacitor 33 varies continuously depending on the control voltage Vt. In contrast, the MOS p-channel transistors Pch1 and Pch2 turn ON or OFF depending on whether the band control signal VS is “L” or “H”, thus increasing or decreasing the capacitance value of the band-switching capacitor 32 by the amount C1. This band-switching capacitor 32 is effective in implementing a voltage controlled oscillator 30 with a broadband range of oscillation frequencies, and is used typically.
The phase noise characteristics of a voltage controlled oscillator 30 with the construction described above are such that optimization of the three-terminal inductor 31, band-switching capacitor 32, continuously variable capacitor 33 and n-channel transistors 34 are related in a complex manner. Therefore, if the band-switching capacitor 32 is switched over a wide range of capacitance values, then the optimal point is shifted in the phase noise characteristics, causing the phase noise characteristics to deteriorate. In order to prevent this deterioration of the phase noise characteristics, two voltage controlled oscillators 30 with different inductance values and the like are prepared, giving a broadband range of oscillation frequencies.
FIG. 16 is a schematic plan view of two conventional three-terminal inductors formed side-by-side and used for the two voltage controlled oscillators of a voltage controlled oscillator apparatus constructed in such manner. In the figure, the three-terminal inductors 31 and 35 are each used in the construction of one of two voltage controlled oscillators that have the construction shown in FIG. 15. The three-terminal inductor 31 is connected to each of the band-switching capacitor 32, continuously variable capacitor 33 and n-channel transistors 34 of one voltage controlled oscillator 30. The three-terminal inductor 35 is connected to each of the band-switching capacitor 36, continuously variable capacitor 37 and n-channel transistors 38 of the other voltage controlled oscillator (not shown). An example of an inductor structure such as the three-terminal inductor 35 formed on a semiconductor substrate is presented in JP-H8-97377A, for example.
In addition, FIG. 17 illustrates a vertical structure wherein the MOS p-channel transistors Pch1 and Pch2 forming the band-switching capacitor 32 shown in FIG. 15 are formed in a semiconductor integrated circuit. An n-well 19 formed on a p-type silicon substrate 18 is divided into regions separated by insulator separation layers 20, and a p-type diffusion layer 21 forming the sources and drains of the MOS p-channel transistors Pch1 and Pch2 is formed in these regions. 22 is a gate electrode made of polysilicon.
When two inductors are formed in a semiconductor integrated circuit to make two voltage controlled oscillators as in the conventional example given above, the chip size becomes large and the cost is high.
In addition, with the structure of the band-switching capacitor 32 shown in FIG. 17, a parasitic diode Di1 arises between the p-type diffusion layer 21 serving as the drains of MOS p-channel transistors Pch1 and Pch2 and the n-well 19 connected to VCC. On the other hand, VCC is applied via a resistor R1 as a DC bias between the transistor Pch1 and a capacitor C1 and between the transistor Pch2 and the capacitor C1, so that when the band control signal VS of the band-switching capacitor 32 is “H” and the MOS p-channel transistors Pch1 and Pch2 are OFF, the amplitude of oscillation centered about VCC appears in the drain between Pch1 and C1 and between Pch2 and C1. As a result, if the amplitude of the voltage controlled oscillator 30 becomes greater than approximately 0.7 V, the same signal is applied to the drains of MOS p-channel transistors Pch1 and Pch2 and the parasitic diode Di1 described above is turned ON, causing a state in which the area between C1 is grounded via the parasitic diode Di1 to VCC. As a result, even if the band control signal VS of band-switching capacitor 32 is switched, the oscillation frequency of the voltage controlled oscillator 30 may not vary adequately or adequate amplitude of oscillation is not obtained, so that the phase noise characteristics may deteriorate.